1. Field of the Invention
The present invention is directed to flexible, optically transparent and conductive coatings and films comprised of carbon nanotubes (CNT) and polymer binders on optically clear substrates such as PET and glass for display and electronic applications, and to the corresponding fabrication methods, coating layer structures and processes. In particular, the invention is directed to fluoropolymer binders applied to the CNT layer to provide protection and enhancement of properties such as increased electrical conductivity, increased optically transparency, moisture resistance, thermal resistance, abrasion resistance and interfacial adhesion.
2. Description of the Background
Transparent and electrically conductive coatings and films are used for versatile applications particularly in flat panel displays, touch screen panel and other electronic applications. These transparent conductors mainly include metal oxides particularly indium-tin oxide (ITO). (See R. G. Gorden, “Criteria for Choosing Transparent Conductors”, MRS Bulletin, Page 52, August/2000). ITO is deposited onto glass and polymer substrates by chemical vapor deposition (CVD), sputtering and other approaches followed by annealing. This offers high electrical conductivity and optical transparency. However, ITO-based coating and film have inferior abrasion resistance and flexibility. The supply of expensive indium is also very limited. Transparent conductive products with easier fabrication and higher performance are in great demand.
Intrinsically conductive polymers such as polyaniline and polythiophene are also used to make flexible transparent conductive coating and films. One significant example in poly(3,4-ethylenedioxythiophene) doped and stabilized with poly(styrenesulfonate) (PEDOT/PSS). (See L. Bert Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, J. R. Reynolds, “Poly(3,4-ethylenedioxythiophene) and Its Derivatives: Past, Present, and Future,” Advanced Materials, Vol. 12, No. 7, pp 481, 2000). However, polymer conductivity and optical transparency are limited. Despite good flexibility, abrasion resistance is also very poor.
Development and application exploration of carbon nanotube have preceded since their discovery in 1991. Nanotubes and nanoparticles are gaining significant attentions due to industrial application potentials. Carbon nanotubes (CNT) of different forms have been discovered including single-wall, dual-wall and multiple-wall. These forms have been incorporated into a number of plastics and films.
Single-wall carbon nanotubes (SWNTs) are manufactured by several methods such as laser ablation, carbon arc and chemical vapor deposition (CVD) including a process using high pressure carbon monoxide (HIPCO). Dilute carbon nanotube dispersions in aqueous or solvent mixtures are referred to as CNT inks.
CNTs include single walled (SWNT), double walled (DWNT) and multi walled carbon nanotubes (MWNT). These forms of CNTs are synthesized by arc-discharge, laser ablation and chemical vapor deposition (CVD), to name a few. (See Carbon Nanotubes Science and Applications; edited by M. Meyyappan, CRC Press, 2004). Carbon nanotubes especially SWNT can also have high electrical and thermal conductivity in addition to good mechanical properties.
Carbon nanotubes are generally mixed with polymers (or monomers followed by polymerization) to form nanocomposites. For example, U.S. Pat. No. 6,265,466 relates to electromagnetic shielding composites comprising nanotubes and polymers. Significant research efforts are focusing on preparation of nanocomposites using this approach. The challenges for this approach include difficulty in uniform mixing due to bundles and agglomeration of CNT, and difficulty in achieving very high conductivity due to an insulative nature of polymers.
Transparent conductive coatings and films can be made by incorporating CNT into clear polymers at a desired thickness (See generally U.S. Pat. Nos. 5,583,887 and 5,908,585). U.S. patent application Ser. Nos. 10/105,623 and 10/442,176 relate to transparent conductive coatings and films with or without certain patterning formed by using single-wall carbon nanotubes (SWNT) through a two step method (e.g. formation of CNT layer via wet process followed by polymer binder coating).
During development of these SWNT based transparent conductive coatings having high conductivity (e.g., 100-105 O/C=), their measured sheet resistance value can fluctuate with changes in time and environment.
This type of CNT network coating on the substrates in sensitive to environmental conditions including moisture and heat. Sheet resistance of a dried bare carbon nanotube coating on the substrates could decrease when first exposed to low moisture level, and then significantly increases at different moisture levels after reaching equilibrium. Sheet resistance also increased upon heating especially at high temperatures such as in the range of 125-400° C. The effects of both moisture and temperature are fully or partially reversible.
When flexible substrates such as plastic films are used, the resulting CNT network coating has very good flexibility. However, these coated substrates often do not have extremely high adhesion and abrasion resistance. Typical substrate types include glass, plastic, ceramic and similar materials.
Currently commercially available transparent conductive coatings and films made from ITO, conducting polymer, and nanocomposites containing nanotubes or other conductive particulates, suffer from at least one common characteristic. All these coating and films are formed as a solid layer to which additional layers of materials can be applied above or below to provided further function or protection from environmental influence. For example, ITO is coated on a flexible transparent polymeric film and over coated with an abrasion resistant polymer such as an acrylic to protect the surface during handling in the factory or by the end user.
A disadvantage is that the acrylic top coating also serves to electrically insulate the coated surface, making contact to the conductive ITO difficult or impossible. Since most commercially available transparent conductive coatings and films are solid materials, the addition of other layers typically interferes with this function of surface conductivity. In the case where composite layers are formed comprising a polymer and a conductive constituent, the polymer in the composite can be selected to provide additional functions such as abrasion, humidity, temperature, adhesion and maintain the conductive properties of the layer. This approach is used commercially to form transparent conductive coatings with PEDOT and polymeric resins to form a solid layer. The disadvantage to this approach is that in these composite coatings, conductivity is greatly reduced by the presence of polymeric resins which serve to dilute and interrupt the conductive pathways.